The invention relates processing of microelectronic substrates and more particularly to an edge extension element aid in such processing.
One of the conditions necessary for the growth of the semiconductor industry is the ability to print ever smaller features on an integrated circuit (IC). However, recently, optical photolithography is facing several challenges which may impede the further development of semiconductor technology. Investments have been made in techniques such as x-ray lithography and electron beam lithography as an alternative to traditional optical lithography. However, optical immersion lithography has garnered interest as potentially meeting the demands of improved semiconductor technology for printing smaller-sized features.
The minimum size W of a feature that may be printed with an optical lithography system is determined by the following equation:W=k1λ/NA   Eq. 1
Where k1 is the resolution factor, λ the wavelength of the exposing radiation and NA is the numerical aperture of the interfacial medium through which the exposing radiation is transmitted at the interface to the feature that is being printed.
As the minimum feature size W has been reduced in the development of semiconductor devices, the wavelength of the exposing radiation has also been reduced. However, the development of new optical exposure sources having further reduced wavelengths faces many challenges, as do improvements in the design of optics required to transmit and focus the light from such reduced wavelength sources.
Looking at Eq. 1 again, it can be seen that the minimum size W is also a function of the numerical aperture NA and becomes smaller when the numerical aperture becomes larger. The numerical aperture is quantified by nsina, where n is the index of refraction of the interfacial medium between the lens and the feature being printed, and a is the acceptance angle of the lens. The sine of any angle is always less than or equal to one and n is approximately equal to one when air is the interfacial medium, so that the numerical aperture cannot exceed one as long as air is the interfacial medium. Replacing air with another medium can increase the effective numerical aperture of the system. In addition to the interfacial medium needing an index of refraction greater than one, such medium should also meet a number of other requirements. For example, the interfacial medium should have a low rate of optical absorption, be compatible and non-contaminating with respect to photoresist and lens materials, and provide a uniform medium. Such requirements appear to be fulfilled by water when the wavelength of the optical exposure source is 193 nm, but a number of practical problems still need to be addressed for optical immersion lithography.
For one, the exposure tool must be able to step quickly from location to location across a wafer to achieve an acceptable rate of throughput. However, rapid motion through a liquid may cause perturbations in the liquid and formation of bubbles. Different approaches have been taken to resolve such problems. However, each such approach has shortcomings. In a first approach, the wafer and the lens are immersed in a pool of water. However, as noted, rapid movement through the water can lead to perturbations and the formation of bubbles which interfere with the quality of the exposure. In another approach, water is dispensed by a nozzle only to the interface between the wafer and the lens and is maintained at such interface by surface tension. Such tools include a device referred to as a “shower head” for dispensing water ahead of the moving lens to a limited area of the wafer and include a vacuum-based or other removal element for removing the wafer from the wafer surface after the lens has passed by.
However, this approach has its own challenges. One such challenge involves the ability to print features disposed at an edge of the wafer. To maximize productivity, the edge of the wafer needs to be fully populated with chips. Immersion lithography tools used in this approach (or possibly others) can have difficulty making lithographic exposures at the edge of the wafer. The immersion liquid may run off the edge of the wafer, or a wafer chuck used to mount the wafer and stop the flow of liquid may interfere with the proper processing of the wafer at the edge of the wafer.
For example, one such technique utilizes a chuck having a raised ring which prevents water from escaping from the chuck.
Similar edge-related problems also exist in chemical-mechanical polishing (CMP), where erosive processes have different rates at the wafer edge than at the center, depending on the geometry of a carrier which holds the wafer, the thickness of the wafer, the age of a film present on the “back” or rear (typically unpatterned) surface of the wafer, differences in intake of moisture intake between the edge and the center of the wafer, among others.
Consequently, a method and structure are desired to address problems of lithographically printing features by optical immersion lithography near an edge of a wafer.